The present invention relates to an optoelectronic scanning apparatus which may be advantageously incorporated into a facsimile transmitter.
In a scanning apparatus of the present type, a light image of a linear portion of an original document is focussed onto a linear photosensor array of the charge accumulation type. Typical of this type of array is a charge coupled device (CCD) array.
In such an array, light incident on photosensor elements of the array causes accumulation of charge in proportion to incident light intensity and time. The output signals of the elements correspond to the accumulated charge. The array also comprises an analog shift register into which the output signals of the elements are transferred in parallel. Main scan pulses cause the signals to be serially fed out of the shift register.
This operation constitutes a main or horizontal scan of the light image of the document. During the main scan the document is moved by one increment perpendicular to the array to effect vertical or subscan.
While the basic prior art apparatus of this type is generally feasible in practical application, a certain amount of distortion is encountered when the apparatus is employed in conjunction with a data compression unit such as a run length encoder. Such a compression unit operates at variable speed which depends on the original document. Where the document contains a large proportion of dark areas, the compression process is relatively slow. For this reason, it is often necessary to interrupt scanning to prevent the data fed from the array into the encoder from exceeding the capacity of the encoder.
When the array is controlled by the encoder to begin or resume scanning, the array is scanned simultaneously with moving the document for vertical scan. As a result, the area covered in the vertical direction by each photosensor element for the current scan line corresponds only to the area of the photosensor element. This differs greatly from continuous scanning where the area covered in the vertical direction by each element corresponds to the product of the width of the element in the horizontal direction and the distance the document is moved relative to the array plus the area of the element. This is because when scanning is begun the output signals of the elements are immediately transferred to the shift register and thereafter fed out. The document is at rest when the transfer is made. Thus, not only is the accumulated charge different from that in continuous scan but image areas between scan lines are not scanned.